We often see that a manufacturer's high-end (D)SLR models offer a "100% viewfinder," which shows the entire exposed frame, and allows precise composition with regard to objects near the edge of the frame.

Lower-end models may instead have a 96% viewfinder, so the exposed frame extends slightly beyond what you see in the viewfinder.

What is the reason for the less-than-100% viewfinder? Why so close but so far? Is it related to pentaprism-vs-pentamirror?